The present invention concerns video interlaced to progressive scan conversion and, in particular, to a method for detecting and correcting errors in the interpolated portions of a progressive image.
Digital television (DTV) signals conforming, for example, to the Advanced Television Systems Committee (ATSC) standard, may have a large number of formats. These formats are typically referenced by the number of horizontal lines in the image and whether each image frame is formed from two fields, each containing alternate lines of the frame (interlaced) or from a single image field containing all of the lines of the frame (progressive). Listed from highest resolution to lowest resolution, the television signal formats defined by the ATSC standard are referenced by the designations, 10801, 720P, 480P and 4801. In these designations, the number refers to the number of horizontal lines in the image and the letter defines the resulting image as being interlaced (I) or progressive (P).
Television receivers that operate according to the standard set by the National Television Standards Committee (NTSC) display 480 lines of active video information as two interlaced fields and, so, have a resolution of 4801. Most of the existing programming in the United States conforms to the NTSC standard.
ATSC television receivers may support many different types of monitors. An ATSC receiver may, for example, be connected to a multi-sync monitor that can adapt to display whatever signal type is being received. This type of multi-sync monitor is typically referred to as a native mode monitor as it allows each possible type of ATSC signal to be displayed at its intended resolution. Alternatively, ATSC receivers may be purchased that can be connected to a standard NTSC monitor. One such receiver is the TU-DST51 DTV Decoder Set-Top Box manufactured by Panasonic. This receiver converts each ATSC signal type into a 4801 output signal that may be displayed on the NTSC monitor. The Panasonic receiver also supports the other types of monitors, automatically converting the received input signal to the format that is supported by the specified monitor.
It is well known that interlaced video signals have artifacts caused by the interlacing of video fields that occur at two different instants. One such artifact is vertical dot crawl. This artifact occurs at vertical edges in the image, typically at edges between portions of the image having different colors. As the name implies, the vertical dot crawl artifact is seen as a line of dots that seem to move from the bottom to the top of the frame. If the display device supports progressive video signals, these artifacts of interlaced scanning may be removed, or at least mitigated, by converting the interlaced video signal to a progressive video signal before it is displayed.
There are many methods for converting an interlaced video signal to a progressive video signal. Typically, interpolated image lines are inserted between the existing lines in each image field of the video signal. These methods are described in a paper by K. Sugiyama et al. entitled xe2x80x9cA Method of De-interlacing with Motion Compensated Interpolation,xe2x80x9d IEEE Transactions on Consumer Electronics, Vol. 45, No. 3, 1999 pp. 611-616.
FIG. 1 is a vertical-temporal graph that illustrates the frame structure of an interlaced video signal. Each of the xe2x80x9cXxe2x80x9ds in FIG. 1 represents a horizontal line of a video image field. Each vertical line of Xs represents an image field. In vertical temporal graphs, the horizontal lines of the video images are shown as perpendicular to the surface of the page and the fields advance across the page from left to right. As can be seen from FIG. 1, the horizontal lines 110, 112, 114, 116 and 118 of the first image field 100 are vertically offset from the lines 120, 122, 124 and 126 of the second image field 102. The combination of the fields 100 and 102 forms the image frame 105.
The interlaced image sequence shown in FIG. 1 is converted to a progressive sequence by adding horizontal lines to each image field as shown in FIG. 2. These added lines are shown in FIG. 2 as xe2x80x9cOxe2x80x9ds. The added lines are positioned between respective pairs of lines in the original field. For example, the lines 111, 113, 115 and 117 are inserted between the original lines 110, 112, 114, 116 and 118 and the lines 121, 123 and 125 are inserted between the original lines 120, 122, 124 and 126 as shown in FIG. 2. These inserted lines may be generated in many different ways, one such method simply repeats the preceding (or succeeding) line in each field. Another method averages successive lines to generate a spatially interpolated line. Both of these methods produce progressive video displays but these displays typically have a lower vertical resolution than a display produced by a true progressive signal having the same number of vertical lines.
Another method for converting an interlaced image into a progressive image combines successive fields, such that the lines from the next field are inserted between the lines of the current field. An alternate method averages fields on either side of the current field to generate temporally interpolated lines. The images produced by these methods typically have better vertical resolution than the spatially interpolated images but exhibit distortion when there is motion among the fields in the image sequence. Several motion-adaptive interpolation techniques have been proposed to eliminate or at least reduce this motion distortion. The simplest method is to apply a motion detector to the image sequence, applying spatial interpolation in regions of the image that exhibit inter-field motion and applying temporal interpolation in regions that do not change from field to field. These techniques, however, provide images having different levels of vertical resolution in different image areas. In addition, the boundary areas between the temporally interpolated pixels and the spatially interpolated pixels may exhibit relatively low-frequency flickering artifacts that detract from the perceived quality of the image.
Motion compensated interpolation is another technique that may be used to generate the interpolated lines in a progressive image. This method divides the image into blocks and, for each block, determines a best matching block from the previous image field, next image field or a combination of the previous and next image fields. The lines of samples in this block are then applied as the interstitial lines in the blocks of the interlaced image to form an equivalent block of a progressive image. While this method produces less distortion related to changes in vertical resolution, it may exhibit another type of distortion, commonly known as blocking distortion, in which the block structure of the image becomes visible in the reproduced image. The block structure becomes visible because the matching blocks for some blocks in the image are better matches than the matching blocks for other blocks of the image.
The present invention is embodied in an apparatus and a method for detecting low-spatial frequency distortion in an interpolated video signal and for generating a compensating signal that, when added to the interpolated video signal reduces the low-spatial frequency distortion.
According to one aspect of the invention, a spatial low-pass filter is applied to corresponding pixels of several adjacent lines of the current field of the original interlaced image. Concurrently, a spatial low-pass filter is applied to corresponding pixels of several interpolated lines that are inserted between the lines of the interlaced image to produce a progressive image. At each pixel position, the low-pass filtered values are compared. If the difference between the low-pass filtered values exceeds a predetermined threshold value, a value approximately equal to the difference between the original pixels and the interpolated pixels is added to the interpolated pixel.